Characteristics of a Subgrid Model for Turbulent Premixed Flames*

A modified version of the linear eddy model (LEM) for subgrid combustion is developed and used for studying the properties of turbulent premixed flames in the core region of Couette flow. The handling of the burning (diffusion/reaction) and specie transport, in this new approach, is fully deterministic. The model is, therefore, found to predict the correct laminar flame speed in the limit of zero turbulence. The effect of three dimensional eddies on scalar fields at the subgrid scales is modeled using a stochastic Lagrangian rearrangement procedure. The derivation of this procedure from the inertial range scaling laws necessitates a calibration procedure since the numerical values of the constants in these laws are unknown. The present LEM approach relates the constant needed to the coefficient of eddy viscosity that is evaluated in a LES, under the assumption of constant turbulent Prandtl and Schmidt numbers. The new approach is found to predict the turbulent flame speed with fair amount of accuracy. The chemistry is modeled using a self propagating scalar field (G-equation) approach and within the limitations of this model, LEM is found to capture the right trends in evolution of flame structure, effects of heat release and turbulence. The encouraging results obtained here warrant further research into extending this subgrid model for cases of finite rate chemistry and inclusion of thermodiffusive mechanisms such as the Lewis number effects.